Re: [PATCH v8 2/6] serial: serialize asynchronous write wakeups with the port lock

[Andy Shevchenko]

On Thu, Oct 29, 2026 at 01:36:58AM -0600, James Hilliard wrote:
> uart_write_wakeup() expects the UART port lock to be held. Several worker,
> DMA completion and interrupt paths call it without that lock, including
> indirect calls from uart_handle_cts_change() and the transmit helpers.
> 
> Take the port lock around these notifications or their transmit helper.
> Keep sleeping SPI and regmap accesses outside the locked region, and keep
> the FSL LPUART completion lock held through its wakeup. Acquire the port
> lock after releasing private channel locks to preserve the ordering used
> by UART callbacks. In the MUX timer, publish available FIFO space under
> the port lock and keep the hardware FIFO-drain wait outside it.
> 
> Use IRQ-safe locking in the Atmel RX and TX tasklets and SB1250 termios
> path, since their interrupt handlers now acquire the same port lock.
> Preserve the caller's interrupt state when releasing those locks.
> 
> On SB1250, lock modem-state updates after reading and filtering the change
> status, and wake modem-status waiters after unlocking. Take the TX lock
> inside its service routine so RX-only interrupts need no acquisition.
> Keep that lock across x_char handling, FIFO consumption and the decision
> to mask TX interrupts: a writer must not enqueue data and enable TX between
> the empty-FIFO check and the interrupt-mask write. Each TX invocation sends
> at most one character.
> 
> Serialize CLPS711x TX servicing and its interrupt-enable state with the
> port lock. Disable TX interrupts without waiting in stop_tx(), since a
> threaded handler may need that same lock. Have an already queued handler
> leave stopped output untouched, and drain it outside the lock during
> shutdown before the transmit FIFO can be released.
> 
> SCCNXP handles interrupts under a controller lock which its UART callbacks
> acquire with the port lock already held. Collect the ports needing wakeups
> in an unsigned long bitmap while servicing the controller, then release
> that lock before visiting the set bits, taking each port lock and
> rechecking the wakeup threshold. Removal drains the IRQ or timer before
> unregistering the ports.
> 
> This serializes write notifications with port-locked serial-core state
> changes, allowing readiness and transmit-storage reservations to gate the
> notification without a check-versus-shutdown race.
> 
> Cover DZ TX servicing and JSM Classic modem notifications as well. On
> DECstation ZS, acquire the port lock before the SCC lock and notify writers
> after releasing the SCC lock. Defer normal TX restart after console output
> to a worker, since console output can already run under the port lock.
> Check transmitter readiness before consuming another character and drain
> the restart worker during shutdown and removal. Release Lantiq's hardware
> lock through the transmit helper's completion hook before notifying the
> line discipline.

...

> static irqreturn_t serial21285_tx_chars(int irq, void *dev_id)

>  	struct uart_port *port = dev_id;
>  	u8 ch;
>  
> +	guard(uart_port_lock_irqsave)(port);

Why _irqsave?

>  	uart_port_tx_limited(port, ch, 256,
>  		!(*CSR_UARTFLG & 0x20),
>  		*CSR_UARTDR = ch,

...

Same Q everywhere (it might be legit in some cases,
it might be too strong in others).

-- 
With Best Regards,
Andy Shevchenko